This invention relates to an input screen of an image intensifying tube and in particular, but not exclusively, to an input screen of a radiographic image intensifying tube (RII tube).
Radiographic image intensifying tubes make it possible to transform a radiographic image into a visible image, generally for the purpose of medical observation.
Such tubes are vacuum tubes comprising an input screen, an electron-optical system and a display or output screen for observing a visible image.
The input screen comprises a scintillator which converts incident X-ray photons into visible photons which then excite a photocathode, generally made from an alkaline antimonide, e.g., cesium-doped potassium antimonide. The photocathode thus excited generates a flow of electrons.
The flow of electrons emitted by the photocathode is then transmitted by the electron-optical system which focuses the electrons and directs them onto the display screen comprising a luminescent substance which then emits a visible light. This light may then be processed, for example, by a television, cinematographic or photographic system.
In the most recent models of such tube, the input screen is comprised of an aluminum substrate covered by a scintillator. The scintillator is itself covered by an electrically conductive layer and which is also transparent to the light emitted by said scintillator. The scintillator may consist of indium oxide, for example. The photocathode is deposited on this transparent layer.
The X-rays strike the input screen on the aluminum substrate side, traverse this substrate, and then reach the material comprising the scintillator.
The luminous photons produced by the scintillator are emitted in substantially all directions. But, in order to increase the resolution of the tube, one chooses in general a substance for the scintillator material such as cesium iodide which has the characteristic feature of growing in the form of crystals that are perpendicular to the surface on which they are deposited. The needle-like crystals which are deposited in this fashion tend to guide the light perpendicularly to the surface, thus favoring good image resolution.
However, due to electron-optical factors, the surface of the input screen is not flat but convex; it may be parabolic or hyperbolic for screens of large dimensions, or, more usually, in the shape of a spherical dome for screens of smaller dimensions.
Due to this curvature of the screen, if the input screen is illuminated by a uniform beam of X-rays, the electron distribution engendered by the screen is not uniform. For example, one can measure the luminosity curve along the diameter of the output screen of the tube for a uniform X-ray illumination of the input screen: the luminosity curve shows the luminous intensity at each point on the diameter of the output screen. It should be noted that this curve is not horizontal; it is generally in the form of an arc of a circle somewhat flattened at the center; the luminosity of the output screen is at a maximum towards the center, but clearly decreases as it approaches the edges. In smaller tubes (15 cm diameter input screen, for example), the decrease of luminosity at the edges, in comparison with the center, is around 25%. In larger screens (30 cm in diameter, for example), the decrease approaches 35%.
It is one object of the invention to provide an image intensifying tube with a more uniform luminosity curve, i.e., one with a smaller spread between the luminosity at the center and the luminosity at the edges, in order to achieve uniform illumination of the input screen. Another object of the invention is to obtain this improved uniformity of luminosity by a simple method that is easier to implement on an industrial scale than the methods proposed in prior art.
Indeed, it may be noted that the prior art (e.g., Ep O 239 991) has already proposed to improve the uniformity of the luminosity by giving a non-uniform distribution to the thickness of the scintillator layer of the input screen. However, this prior art method is not easy to implement for the following reason: the efficiency of the scintillator increases and then decreases with the thickness; in order to obtain a satisfactory efficiency, it is necessary to start at the maximum level, but one is then on a plateau of the efficiency curve as a function of thickness, and therefore the thickness must be varied considerably in order to modify luminosity. From this it results that a high degree of uniformity in scintillator thickness must be maintained and this is industrially impractical, all the more because the scintillator is deposited in a very thick layer (on the order of 400 micrometers).
It should be noted that elsewhere in the prior art (e.g., EP A 0 378 257) it has been proposed to add a selectively absorbent layer between the scintillator and the photocathode. The function of this layer is to absorb light wavelengths emitted by the scintillator below a certain wavelength because these wavelengths are interfering, and to allow preferred wavelengths to pass freely to the photocathode. This layer may be of variable thickness so that the optical absorption at the center may be greater than the absorption at the edges. The greater absorption is due to the longer optical path to be traversed by the light rays emitted by the scintillator through this absorption layer. In order to obtain this effect, a thickness varying from 10 to 20 microns is indicated for the absorption layer.